1. Field of the Invention
The present invention relates to high frequency signal sources and more particularly to a high frequency source having low spurious noise output. The invention further relates to a high frequency source wherein the output of a Digital-to-Analog stage of a direct digital synthesizer is mixed with a reference frequency that is a multiple of the Digital-to-Analog sampling rate.
2. Background of the Art
Highly adjustable or tunable frequency sources or drivers are required for many advanced communication systems which employ high frequency FM modulation to transfer high bit rate digital data. A variety of frequency synthesizers have been developed to meet this need as reference signal sources or oscillators in digital communication systems.
However, as newer communication systems are developed for serving larger numbers of users, frequency resolution and noise generation become increasingly significant problems. This is a result of the narrower band width channel restrictions imposed on such systems to accommodate additional users within given bandwidth allocations, which results in continually smaller separations between adjacent channels. Therefore, to properly maintain minimum interference and maximum isolation in this environment, reference or mixing frequencies for each channel must be resolved with increasingly high accuracy and resolution, and minimum noise.
Direct digital synthesizers are finding extensive use in advanced digital communication systems especially for generating variable reference frequencies required by frequency hopping and high volume, multi-channel, systems. Direct digital synthesizers offer relatively high frequency accuracy and resolution, efficient interface with digital control circuitry and logic, and provide high speed operation and low power consumption, all of which are a must in satellite and mobile communication systems.
Direct digital frequency synthesizers typically comprise a digital phase accumulator, a periodic wave function conversion element, in the form of Read Only Memory (ROM) devices, and a Digital-to Analog Converter (DAC). The phase accumulator is used for incrementing a phase angle which is applied at regular sampling intervals to the conversion element which converts accumulated phase angles to a sine function amplitude which is then converted into an analog signal. That is, the instantaneous amplitude at given points during the period of the sine function are computed as digital values from accumulated phase and then transferred to the digital-to-analog converter for conversion to an analog signal having the same frequency as the phase angle data.
When digital information, such as a sine amplitude, is converted to analog form, spurious noise is created due to a quantization effect. This well known effect for any digital to analog conversion process creates spurious noise on a periodic basis.
While direct digital synthesizers provide high accuracy, reproducible, frequency tuning across a wide range of frequencies they are generally restricted to lower frequencies. The output frequency is limited to 1/2 the sampling frequency and the phase increment generation and conversion cannot be sampled with accuracy and high resolution above about 40-60 MHz. Therefore, direct digital synthesizers provide a primary or fundamental frequency output tunable from as low as 0 Hz up to a maximum number on the order of 10-30 Mhz.
Unfortunately, this upper frequency limit is too low for modern communication systems which range from 100-200 MHz into the gigahertz (GHz) range for modern satellite transponders. Many communication systems, especially those still in the planning stages, require very high frequency sources operating at many gigahertz. Since this is well beyond the fundamental frequency of direct digital synthesizer designs, some form of up conversion is generally performed using other circuit elements.
To provide higher frequencies, the fundamental output frequency from the direct digital synthesizer is transferred into a signal mixer where it is mixed with a high frequency reference signal. This mixing generates a sum and difference between the two signals and the reference signal and the desired frequency signal is subsequently transferred from the mixer to other apparatus for use. Therefore, an independent direct digital synthesizer circuit or circuit module is connected to an independent reference source and mixer circuit for production of the final frequency desired. The reference is chosen to bring the direct digital synthesizer output frequency up to the frequency range of interest and the tunable direct digital synthesizer output acts as a fine tuning source for the reference. Therefore, both high frequency and accurate tuning would be obtained.
However, the output of direct digital synthesizers are known to have spurious noise or noise spurs at various frequencies adjacent to the desired primary output frequency due to the phase conversion process. Likewise any reference signal may also have noise components. During mixing these various noise components may add together and generate a variety of unwanted frequencies at significant power levels in the final output signal. Therefore, the resolution of the up-converter output is often poor. The more stringent resolution requirements imposed on advanced systems are not easily satisfied by this up-conversion process.
An alternative is to take advantage of the [Sin X]/X nature of the typical direct digital synthesizer output and use the higher order alias frequencies generated by the direct digital synthesizer. That is, higher frequency components are generated by the direct digital synthesizer and occur at various harmonic multiples of the direct digital synthesizer sample clock mixed with the fundamental output. It has been thought that a bandpass filter could be used to select these high frequency components for amplification by a broad band power amplifier and use for the final signal. However, noise spurs present at lower frequencies are also present at higher frequencies and gain in relative magnitude with respect to the desired output frequencies as the frequency increases. Extremely high resolution filtering is, therefore, needed to remove such noise, which is on the order of the same magnitude as the desired output and close in frequency. This type of filtering is not practical in commercial applications due to cost and high complexity.
What is needed is a simple and effective way of converting a high resolution, high accuracy, direct digital synthesizer output from low frequency to high to extremely high frequencies without adding noise or otherwise degrading the output with spurious noise and spectral spurs. It would also be helpful if this conversion can be accomplished in a very cost effective and low power consumption manner to aid in use in mobile or portable communication equipment.